Detect if a script is being run via shebang or was specified as a command line argument

Question

In the Pyenv project, we've had a peculiar problem.

We are substituting python (and python*) with our Bash scripts ("shims") that select a Python executable to run at runtime.

Now, some users wish to use a special selection logic when a Python script is run as path/to/script.py. The problem is, this logic should NOT apply if the script is instead run as <python> path/to/script.py!

Is there a way to reliably distinguish these two cases?

I wasn't able to find anything: depending on the way command line arguments are formulated in the 2nd case, the exact same command line could be executed in both cases:
_{(the Bash script given is not a real shim, just a demonstration example to showcase what our logic sees and does)}

$ cat python3 
#!/bin/bash
echo "'$0'"
for a in "$@"; do
  echo "'$a'"
done
# need to do the detection here
exec python3 "$@"

$ cat t.py 
#!/home/vmuser/python3
import sys
print(sys.argv)

$ $PWD/t.py
'/home/vmuser/python3' 
'/home/vmuser/t.py'
['/home/vmuser/t.py']

$ $PWD/python3 $PWD/t.py 
'/home/vmuser/python3'
'/home/vmuser/t.py'
['/home/vmuser/t.py']

Since shebang is a Linux kernel feature -- maybe it sets some indicator that this mechanism has been used?

---

We've considered requiring users to use a special shebang in their Python scripts that they wish to apply the special logic to, but that idea proved unpopular because it makes those scripts unportable.

Answer 1

Since shebang is a Linux kernel feature -- maybe it sets some indicator that this mechanism has been used?

Yes, it does. Linux sets the AT_EXECFN auxiliar vector entry to the path of the original executable. In C, you can do it with char *at_execfn = (char*)getauxval(AT_EXECFN), followed by stat(at_execfn), etc.

Getting it from bash is tricky, though. You can try unpacking the /proc/self/auxv and then looking through /proc/self/mem. Good luck with that ;-)